kcm: Check if sk_user_data already set in kcm_attach

This is needed to prevent sk_user_data being overwritten.
The check is done under the callback lock. This should prevent
a socket from being attached twice to a KCM mux. It also prevents
a socket from being attached for other use cases of sk_user_data
as long as the other cases set sk_user_data under the lock.
Followup work is needed to unify all the use cases of sk_user_data
to use the same locking.

Reported-by: syzbot+114b15f2be420a8886c3@syzkaller.appspotmail.com
Fixes: ab7ac4e ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: Tom Herbert <tom@quantonium.net>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/kcm/kcmsock.c

@@ -1410,18 +1410,30 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
 		return err;
 	}
 
-	sock_hold(csk);
-
 	write_lock_bh(&csk->sk_callback_lock);
+
+	/* Check if sk_user_data is aready by KCM or someone else.
+	 * Must be done under lock to prevent race conditions.
+	 */
+	if (csk->sk_user_data) {
+		write_unlock_bh(&csk->sk_callback_lock);
+		strp_done(&psock->strp);
+		kmem_cache_free(kcm_psockp, psock);
+		return -EALREADY;
+	}
+
 	psock->save_data_ready = csk->sk_data_ready;
 	psock->save_write_space = csk->sk_write_space;
 	psock->save_state_change = csk->sk_state_change;
 	csk->sk_user_data = psock;
 	csk->sk_data_ready = psock_data_ready;
 	csk->sk_write_space = psock_write_space;
 	csk->sk_state_change = psock_state_change;
+
 	write_unlock_bh(&csk->sk_callback_lock);
 
+	sock_hold(csk);
+
 	/* Finished initialization, now add the psock to the MUX. */
 	spin_lock_bh(&mux->lock);
 	head = &mux->psocks;